High availability is not disaster recovery
Both availability and disaster recovery rely on some of the same best practices, such as monitoring for failures, deploying to multiple locations, and automatic failover. However, Availability focuses on components of the workload, whereas disaster recovery focuses on discrete copies of the entire workload. Disaster recovery has different objectives from Availability, measuring time to recovery after the larger scale events that qualify as disasters. You should first ensure your workload meets your availability objectives, as a highly available architecture will enable you to meet customers’ needs in the event of availability impacting events. Your disaster recovery strategy requires different approaches than those for availability, focusing on deploying discrete systems to multiple locations, so that you can fail over the entire workload if necessary.
You must consider the availability of your workload in disaster recovery planning, as it will influence the approach you take. A workload that runs on a single Amazon EC2 instance in one Availability Zone does not have high availability. If a local flooding issue affects that Availability Zone, this scenario requires failover to another AZ to meet DR objectives. Compare this scenario to a highly available workload deployed multi-site active/active, where the workload is deployed across multiple active Regions and all Regions are serving production traffic. In this case, even in the unlikely event a massive disaster makes a Region unusable, the DR strategy is accomplished by routing all traffic to the remaining Regions.
How you approach data is also different between availability and disaster recovery. Consider a storage solution that continuously replicates to another site to achieve high availability (such as a multi-site, active/active workload). If a file or files are deleted or corrupted on the primary storage device, those destructive changes can be replicated to the secondary storage device. In this scenario, despite high availability, the ability to fail over in case of data deletion or corruption will be compromised. Instead, a point-in-time backup is also required as part of a DR strategy.
